1. Field of Invention
This invention relates to non-contact voltmeters, and to the indication of atmospheric charges due to earth-air currents, as currents of an electrical nature in bodies of water will also be impressed upon any organisms, i.e. fish, in the water, and electrical stimulus of these organisms will result.
2. Description of Prior Art
The total of earth-air currents worldwide is accepted to be in the range of 1800 amps. These currents maintain the existence of earth's electrostatic field, which varies from 100 to 1500 volts per meter near the earth's surface. Atmospheric charges and currents vary locally, and are influenced by cloud heights, moisture content, thunderstorms, jet streams, wind, ionization due to solar radiation, conductivity of the location's soil or water, and natural or man-made topography or structures. These currents will also be expressed in the earth's uppermost surface, and seek paths of least resistance. These may be natural or man-made paths not limited to metallic buried bodies such as pipelines, ore deposits, soils rich in ionic chemistry, and any body of water in conductive contact with the earth's surface.
Naturally occurring electrical currents are greatest in the vicinity of thunderstorm activity, and certain levels of current peripheral to these storms are associated with air inflowing to updrafts, which results in strong winds and gusts depositing insects into the water, and association may be made between these conditions and the feeding of fish.
A British patent of 1863 recognized electrofishing, in which fish are attracted to an anode conducting a direct current. Electrical stimulus of fish in their natural environment is recognized in U.S. Pat. No. 5,445,111 (1995) to Smith, in which fish avoid a barrier presented by an electrical current impressed in the water.
Non-contacting voltmeters are used in many applications to non-intrusively measure electrostatic charges between bodies or surfaces. The prior art has employed various methods to provide sufficient sensitivity to measure small charges. The earlier devices used vacuum tubes, as exemplified by Ecker et al. U.S. Pat. No. 2,927,269 (1960). This voltage-measuring device requires ionization of the air at the probe by radium, which provides sufficient conductivity to complete a circuit through which enough current will flow to the ground to permit meter indication. Ecker's device uses one of two possible input resistances, in parallel with the control element, either of which were considered necessary in design, in which electrons collecting at the grid could leak off to ground. Without a return path to ground for electrons, tube conduction stops as the grid becomes more and more negative. This is true when tubes are operated at normal accepted cathode temperatures and plate voltages. The current through a tube is then so great, that a grid leak resistor is necessary to make the tube and circuit stable, which decreases input sensitivity in that it provides a leakage path to ground for the desired signal. The voltage developed across the grid leak, applied to the control element, is not the same voltage existing at the probe, but a representation of the voltage derived as a result of current flow through the resistance. Ecker's device also uses a filament cathode, in which the electrons are emitted directly by a glowing wire, and undesirable current variations in the filament circuit will deflect meter indication as readily as a desired signal. Other devices, such as L L Blackwell et al, U.S. Pat. No. 3,449,668 (1969), use a radioactive ionizing element, and two directly heated vacuum tubes arranged in a bridge circuit, but do not reduce cathode temperature to secure higher sensitivity or use a circuit providing voltage amplification.
Later non-contact voltmeters use an input circuit based on a Field-Effect transistor (FET), often a metal-oxide silicon type (MOSFET), in which the gate of the device capacitively couples the signal of interest through a layer of silicon dioxide 1000 angstroms thick. This thin layer is easily ruptured by as little as 100 volts, and typically is protected by one or more zener diodes in parallel with the gate, either integrally in manufacture, or incorporated into the circuit design. This degrades the input of the MOSFET in several ways: the leakage current is greater, and the input resistance is less, because there is a diode across the input. Leakage currents of 10 sup−10 amps and input resistances of 10 sup 11 ohms are typical for diode gate protected MOSFETs. Unprotected MOSFETs are unsuited to direct application of atmospheric potentials due to the likelihood of gate rupture from electrical discharges. Another disadvantage of the MOSFET is that it does not introduce the input signal directly into the stream of current to be modulated, but relies upon opposing charges developed through a capacitor formed by the silicon dioxide layer. These charges, a representation of the input signal, modify current-carrier mobility within a conduction channel. This capacitance introduces a 90-degree phase shift in any AC signal, and isolates a DC signal. Typically, an n-channel depletion type MOSFET is used in order to permit sensing voltage which may be negative or positive. This type of FET has a nominally conductive channel of n-type material between its drain and source. Since the current in a FET is due to the majority carriers (electrons for an n-type material), introducing a negative voltage to the gate induces positive charges into the conduction channel, which reduces the availability of majority carriers, decreasing conductivity. Such capacitive coupling is avoided in a vacuum tube; the actual input signal may be introduced upon a conductive path to a point directly within the current to be modulated.
Voltmeters using solid state input devices suffer from an effect referred to as “dead band.” This is a range between a small negative voltage, through a zero-voltage point, to a small positive voltage, within which input current flow to a measuring device is insufficient to distinguish it from internal leakage currents due to input protection or leakage of the measuring device itself. In Govaert, U.S. Pat. No. 4,950,978 (1990) conductivity of the air in the vicinity of the probe is increased by application of high voltage AC coupled to the probe. This creates ion pairs providing enough conductivity for a current to flow, sufficient to be measured by the sensing electronics. The applied high voltage AC, corona discharge, and recombining ions and electrons create a large AC component, which must be filtered before the DC component can be isolated. This design also uses a resistance in parallel with the input, decreasing the sensitivity, and making the input voltage an analog, rather than the actual input signal.
Other electrostatic voltmeters apply a high-frequency modulation to a capacitance between the metering apparatus and the test surface to be measured.
The disadvantages in this method are that as the modulation frequency is increased, smaller samples are collected for measurement. Low voltages become increasingly hard to measure since they induce little current into a sampling capacitance. The resultant sample is a quantized charge, a function of the capacitance and the stability of the modulation and the frequency used. Such a sample is derivative, not the actual input signal. This method is also somewhat intrusive as the modulating frequency imposed into a subject may result in rectification, resulting in a change of a DC level to be measured.